This is a short excerpt from the California Penal Code, section 12276.1. It does not address shotguns or handguns; I will correct this deficiency later.

Penal Code Section 12276.1 [abbreviated; rifle section only]:
"Assault weapon" shall mean any of the following:
A semiautomatic, centerfire rifle that has the capacity to accept a detachable magazine and any one of the following:
A.A pistol grip that protrudes conspicuously beneath the action of the weapon.
B.A thumbhole stock.
C.A folding or telescoping stock.
D.A grenade launcher or flare launcher.
E.A flash suppressor.
F.A forward pistol grip.
A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.
A semiautomatic, centerfire rifle that has an overall length of less than 30 inches.